Enamels used in the painting of automobiles are typically based on low molecular weight polymers which require lower concentrations of solvent than the higher molecular weight polymers used in lacquers. In enamel compositions, the low molecular weight polymer is crosslinked after application to increase the molecular weight and durability of the cured finish.
It has previously been recognized that amine groups on acrylic resins can be used in the required crosslinking reactions. Amine-substituted polymers have been prepared in the past by first preparing a carboxylic acid functional polymer which is subsequently modified to yield a polymer having primary or secondary amine groups. Such a technique is described, for example, in Den Hartog et al., U.S. Pat. No. 4,525,521. Den Hartog et al. also describes a process in which glycidyl methacrylate or glycidyl acrylate is reacted with ammonia to yield a polymer having an alcohol and primary amine.
The reaction technique of Den Hartog et al. is complicated by the side reactions between the primary amine groups and epoxy groups before all of the epoxy groups have been consumed. This typically results, in the absence of a high concentration of ammonia and the attendant high pressures, in gelation of the acrylic polymer during the reaction.